1. Abt, A. 1952,Astrophys. J.,115, 199.
2. Allen, W. 1973,Astrophysical Quantities, 3 edn, University of London, Athlone Press.
3. Bappu, M. K. V., Raghavan, N. 1969,Not. R. astr. Soc,142, 295.
4. Becker, W., Fenkart, R. 1970, inIAUSymp 38: The Spiral Structure of our Galaxy, Eds W. Becker and G. Contopoulos, D. Reidel, Dordrecht, p. 205.
5. Bell, R. A., Eriksson, K., Gustafsson,B Nordlund, A. 1976,Astr. Astrophys. Suppl. Ser.,23, 37.